The most popular open source webmail solution: SquirrelMail. SquirrelMail is known for its low system requirements and scalability, plus it's easily extendable through the plugins-mechanism.
Commercial Use
Modify
Distribute
Place Warranty
Sub-License
Hold Liable
Distribute Original
Disclose Source
Include Copyright
State Changes
Include License
These details are provided for information only. No information here is legal advice and should not be used as such.